1. Field of the Invention
This invention relates to a method for fabricating domain reversals in a predetermined pattern on a ferroelectric substance possessing a nonlinear optical effect in order to produce an optical wavelength converter element for converting a fundamental wave into a second harmonic wave and, more particularly, a method for fabricating domain reversals in which a substrate made of LiNbO.sub.3 or LiTaO.sub.3 and doped with MgO is used as the ferroelectric substance.
2. Description of the Prior Art
A proposal has already been made by Bleombergen et al. in Physics Review vol. 127, No. 6 in 1918 (1962), in which the wavelength of a fundamental wave is converted into a second harmonic wave using an optical wavelength converter element with regions (domains) where the spontaneous polarization of a ferroelectric substance possessing a nonlinear optical effect are periodically inverted.
In this method, the fundamental wave can be phase matched with the second harmonic wave by setting the period .LAMBDA. of the domain reversals to be an integral multiple of a coherence length .LAMBDA.c which is given by EQU .LAMBDA.c=2.pi./{.beta.(2.omega.)-2.beta.(.omega.)} (1)
where .beta.(2.omega.) designates the propagation constant of the second harmonic wave, and 2.beta.(.omega.) represents the propagation constant of the fundamental wave. When wavelength conversions are effected using the bulk crystal made of a nonlinear optical material, a wavelength to be phase-matched is limited to a specific wavelength inherent to the crystal. However, in accordance with the above described method, phase matching can be efficiently carried out by selecting a period .LAMBDA. which satisfies the condition (1) for an arbitrary wavelength.
As described in the Applied Physics Letter Vol. 59 (21), Nov. 18, 1991, pp. 2657-2659, it has been heretofore known that LiNbO.sub.3 (MgO--LN) doped with MgO is preferably used as a ferroelectric substance which constitutes aforementioned periodic domain reversals. Practically, this MgO--LN has an optical damage threshold value which is higher by two or more decimal points when compared with LiNbO.sub.3 which is not doped with MgO. Hence, when periodic domain reversals are defined on this MgO--LN, there will be obtained an optical wavelength converter element which produces a wavelength-converted wave having a high intensity with a significantly high efficiency of wavelength conversion.
Similarly, LiTaO.sub.3 (MgO--LT) doped with MgO has also been known as a ferroelectric substance which is suitable for the production of periodic domain reversals. Various attempts have already been made to fabricate an optical waveguide type or bulk crystal type wavelength converter element with the use of these ferroelectric substances.
Specifically, a method in which when a softened MgO--LN ingot is extended to produce a fiber, the area which is being extended is locally exposed to a laser beam so as to cause the spontaneous polarization of that area to be inverted is known as a method for fabricating periodic domain reversals on the foregoing MgO--LN and the MgO--LT.
However, according to the above-mentioned method, it is impossible to fabricate periodic domain reversals on MgO--LN and MgO--LT in the shape of a substrate. When optical waveguide type optical wavelength converter elements are fabricated, it is necessary to produce periodic domain reversals on a substrate made of such ferroelectric substances. Meanwhile, the majority of bulk crystal type wavelength converter elements is also formed from a substrate made of a ferroelectric substance. Therefore, the above-mentioned conventional method can be considered least valuable in practical use since it cannot fabricate domain reversals on a ferroelectric substance in the form of a substrate.
Another widely known technique for fabricating periodic domain reversals on a substrate of a ferroelectric substance is that the surface of a substrate is covered with a periodic electrode mask, and this substrate is subjected to the application of an electric field. However, according to such a technique, when a LT substrate is used, it is necessary to apply an electric field as large as 200 kV/cm or thereabouts to the substrate, and hence this tends to involve the destruction of crystals of the ferroelectric substance. Thus, this method suffers from a problem that it is difficult to define the periodic domain reversals in a predetermined pattern in a highly controllable manner. This method also suffers from a drawback that it is difficult to deeply fabricate domain reversals.